You can do that simply by editing the built-in reports:
- under Reports, search for "Disk: Servers less than 1 GB free HD" or "Disk: Workstations less than 1 GB free HD"
- hit Edit Report
- hit Save as and save the report under a new name
- in the middle of the screen, search for the element with the alias "free"
- under column Criteria, change the filter criterion, i.e. to 10024 or 20048
- hit Save & Run